Font size: "base" font size of your WebStore. By default set to 16, with all the other font sizes being relative to it. Some custom fonts may require increasing the size for better legibility. Try and see what works the best for you.
Gap: gap is the space between most of the elements — like products on your listing pages, distance between elements in the header or even the "gap" between footer columns, or extra padding on content pages.
Rounded corners: for a softer look of your WebStore (and mostly buttons on it) you can increase that number. We would recommend values between 0 (square corners) and 25 (would make the buttons rounded on both sides, but the rounder-corner-boxes wouldn't look odd).
Container size: determines the width of the WebStore's content. Carousels, product pages, content pages — everything is constrained to maximum of that width (on desktop).
Newsletter box placement: checking the box will give the Newsletter sign-up form it's own section on the homepage instead of just a column in a footer. Note: Needs the Newsletter enabled in the settings first!